U.S. patent number 6,266,108 [Application Number 09/046,765] was granted by the patent office on 2001-07-24 for reflective liquid crystal display device with a panel, a light guide plate and polarizing plate.
This patent grant is currently assigned to Sony Corporation. Invention is credited to Yang Ying Bao, Takayuki Fujioka, Hideo Kataoka, Yukio Kinoshita, Tetsuo Urabe.
United States Patent |
6,266,108 |
Bao , et al. |
July 24, 2001 |
Reflective liquid crystal display device with a panel, a light
guide plate and polarizing plate
Abstract
A reflective display device includes a panel, a light guide
plate and a light source. The panel is provided with a transparent
first substrate lying on the side of the external incident light, a
second substrate joined to the first substrate with a predetermined
gap therebetween and lying on the reflection side, a guest-host
liquid crystal layer held in the gap between the substrates, and
electrodes provided on each substrate for applying a voltage to the
guest-host liquid crystal layer. The light guide plate is composed
of a transparent material and is arranged on the outside of the
first substrate. The light source is arranged on the end of the
light guide plate and generates illumination light as required. The
light guide plate normally transmits external light onto the first
substrate and emits the external light reflected from the second
substrate, and also, as required, guides illumination light onto
the first substrate and emits the illumination light reflected from
the second substrate.
Inventors: |
Bao; Yang Ying (Kanagawa,
JP), Urabe; Tetsuo (Kanagawa, JP),
Kinoshita; Yukio (Kanagawa, JP), Kataoka; Hideo
(Kanagawa, JP), Fujioka; Takayuki (Kanagawa,
JP) |
Assignee: |
Sony Corporation (Tokyo,
JP)
|
Family
ID: |
26431639 |
Appl.
No.: |
09/046,765 |
Filed: |
March 24, 1998 |
Foreign Application Priority Data
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Mar 25, 1997 [JP] |
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9-090135 |
Sep 10, 1997 [JP] |
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9-262876 |
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Current U.S.
Class: |
349/63; 349/113;
349/61; 349/62; 349/67 |
Current CPC
Class: |
G02B
6/003 (20130101); G02B 6/0048 (20130101); G02B
6/0038 (20130101); G02F 1/13725 (20130101); G02F
1/133615 (20130101); G02B 6/0056 (20130101); G02B
6/0071 (20130101); G02F 1/133616 (20210101); G02F
1/133553 (20130101); G02F 1/133502 (20130101) |
Current International
Class: |
G02F
1/1335 (20060101); G02F 1/139 (20060101); G02F
1/13 (20060101); G02F 001/335 () |
Field of
Search: |
;349/61,62,67,63,70,113 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 470 817 A2 |
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Feb 1992 |
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EP |
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0 545 705 A1 |
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Jun 1993 |
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EP |
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0 737 882 A2 |
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Oct 1996 |
|
EP |
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WO 93/16410 |
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Aug 1993 |
|
WO |
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WO 95/34009 |
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Dec 1995 |
|
WO |
|
Primary Examiner: Sikes; William L.
Assistant Examiner: Chowdhury; Tarifur R.
Attorney, Agent or Firm: Sonnenschein, Nath &
Rosenthal
Claims
What is claimed is:
1. A front lit reflective display device, comprising:
a panel comprising a transparent first substrate lying on a side of
external incident light, a second substrate joined to said first
substrate with a predetermined gap therebetween and lying on a
reflection side, an electro-optical material held in said gap, and
an electrode provided on at least one of said first substrate and
said second substrate for applying a voltage to said
electro-optical material and a reflecting layer provided on said
second substrate;
a transparent light guide plate arranged on an outside of said
first substrate, said light guide plate normally transmitting
external light onto said first substrate and emitting the external
light reflected from said second substrate, while, as required,
guiding illuminating light onto said first substrate and emitting
the illumination light reflected from said second substrate;
and
a light source arranged on an end of said light guide plate, and
wherein a polarizing plate is provided between the light source and
said light guide plate for converting the unpolarized illumination
light radiating from said light source into linearly polarized
light, and leading it onto said light guide plate, and suppressing
undesired scattering of the illumination light inside said light
guide plate,
wherein said light guide plate and said panel are joined to each
other with a transparent intervening layer therebetween, said
intervening layer comprising a transparent resin having an adhesion
and a refractive index being approximately set so as to suppress
undesirable reflection of the illumination light and the external
light at the interface between said light guide plate and said
panel,
wherein said light guide plate is provided with a groove on the
back surface thereof for preventing said resin of said intervening
layer from leaking out to a side or outer surface of said light
guide plate and provides a uniform coverage of resin within an
interface between the light guide plate and the panel, when the
back surface of said light guide plate and the surface of said
panel are joined to each other, and
wherein said electro-optical material comprises a liquid crystal
which can be controlled in an alignment direction parallel to or
orthogonally to a polarization direction of said illumination light
converted into linearly polarized light.
2. A reflective display device according to claim 1, wherein said
reflective display device further comprises a collimating means for
collimating the illumination light radiating from said light source
and leading it perpendicularly onto the end of said light guide
plate.
3. A reflective display device according to claim 2, wherein said
light source is semicylindrically formed and arranged facing the
end of said light guide plate and said collimating means
corresponds to a semicylindrical collimator lens arranged between
said light source and said light guide plate.
4. A reflective display device according to claim 1, wherein said
polarizing plate converts illumination light into linearly
polarized light which is parallel to or perpendicular to said light
guide plate.
5. A reflective display device according to claim 1, wherein said
light guide p late comprises a trapezoidal section divided into ban
ds and an inclined step lying between each band of said trapezoidal
section, each band of said trapezoidal section comprising a curved
lens area, and said light guide plate reflects the illumination
light radiating from said light source at each step so as to guide
it onto said first substrate and emits the illumination light
reflected from said second substrate through said lens area of each
band of said trapezoidal section.
6. A reflective display device according to claim 1, wherein said
panel comprises a guest-host liquid crystal layer as said
electro-optical material, comprising a liquid crystal as a host to
which a dichroic dye is added as a guest.
7. A reflective display device according to claim 6, wherein said
panel comprises a reflecting layer lying on the side of said second
substrate for scattering and reflecting external light, and a
quarter-wavelength layer provided between said guest-host liquid
crystal layer and said reflecting layer.
8. A reflective display device according to claim 1, wherein said
panel comprises a polarizing plate provided on the side of said
first substrate and a liquid crystal layer as said electro-optical
material, said liquid crystal layer functioning as a
quarter-wavelength plate in response to the state of an applied
voltage.
9. A reflective display device according to claim 1, wherein a
quarter-wavelength plate is provided between said polarizing plate
and said liquid crystal layer, and said liquid crystal layer
comprises a nematic liquid crystal layer having a positive
dielectric anisotropy and a twisted alignment, said quarter
wavelength plate functions as a quarter-wavelength plate in the
absence of an applied voltage, and does not function as a
quarter-wavelength plate in the presence of an applied voltage.
10. A front lit reflective display device, comprising:
a panel comprising a transparent first substrate lying on a side of
external incident light, a second substrate joined to said first
substrate with a predetermined gap therebetween and lying on a
reflection side, an electro-optical material held in said gap, and
an electrode provided on at least one of said first substrate and
said second substrate for applying a voltage to said
electro-optical material and a reflective layer provided on said
second substrate;
a transparent light guide plate arranged on an outside of said
first substrate, said light guide plate normally transmitting
external light onto said first substrate and emitting the external
light reflected from said second substrate, while, as required,
guiding illumination light onto said first substrate and emitting
the illumination light reflected from said second substrate;
and
a light source arranged on an end of said light guide plate,
wherein said light guide plate comprises a plurality of planar
sections divided into bands and an inclined step lying between each
band of substantially parallel planar sections and said light guide
plate reflects the illumination light guided forward at each step
so as to guide it onto said first substrate and also emits the
illumination light reflected from said second substrate through
said planar sections,
wherein said light guide plate and said panel are joined to each
other with a transparent intervening layer therebetween, said
intervening layer comprising a transparent resin having an adhesion
and a refractive index being approximately set so as to suppress
undesirable reflection of the illumination light and the external
light at the interface between said light guide plate and said
panel,
wherein said light guide plate is provided with a groove on the
back surface thereof for preventing said resin of said intervening
layer from leaking out when the back surface of said light guide
plate and the surface of said panel are joined to each other,
and
wherein said electro-optical material comprises a liquid crystal
which can be controlled in an alignment direction parallel to or
orthogonally to a polarization direction of said illumination light
converted into linearly polarized light.
11. A reflective display device according to claim 10, wherein said
step of said light guide plate inclines in the range from 40 to 50
degrees toward said planar section.
12. A reflective display device according to claim 10, wherein said
step comprises a curved inclined area for reflecting illumination
light diffusively so as to lead it onto said first substrate.
13. A reflective display device according to claim 10, wherein each
said step is formed so as to have a different angle of inclination
for reflecting illumination light in accordance with the angle of
inclination and leading it onto said first substrate at a different
angle.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reflective display device to
perform a display by using external light such as natural light,
and more specifically, it relates to an illuminating structure of a
reflective display device, which is used as an auxiliary when
external light is scarce.
2. Description of the Related Art
Among current various modes of display devices, mainly adopted are
a TN mode or an STN mode in which a nematic liquid crystal having a
twisted or super twisted alignment is used. However, these modes
require a pair of polarizers for operation, and because of the
light absorption thereof, they have a low transmittance, incapable
of achieving a bright display screen. In addition to the above
modes, a guest-host mode which uses a dichroic dye has been
developed. A liquid crystal display device having a guest-host mode
takes advantage of the anisotropy of the absorption coefficient of
the dichroic dye added to the liquid crystal, in order to perform
the display. By using a rod-shaped dichroic dye, the alignment
direction of the dye changes as the molecular alignment of the
liquid crystal is changed by applying a voltage to the electric
field since the molecules of the dye are aligned in parallel to the
molecules of the liquid crystal. The dye does or does not develop a
color depending on the direction, and therefore by applying a
voltage, the coloring mode of the liquid crystal display device can
be switched.
FIG. 5A and FIG. 5B show a HEILMEIER type guest-host liquid crystal
display device. FIG. 5A shows the state in the absence of an
applied voltage, while FIG. 5B shows the state in the presence of
an applied voltage. This liquid crystal display device includes a
p-type dye and a nematic liquid crystal having a positive
dielectric anisotropy (N.sub.p liquid crystal). The p-type dichroic
dye having an absorption axis which is substantially parallel to
the molecular axis, strongly absorbs the polarization component Lx
which is parallel to the molecular axis, and hardly absorbs the
polarization component Ly which is perpendicular to it. In the
state shown in FIG. 5A when no voltage is applied, the polarization
component Lx included in the incident light is strongly absorbed by
the p-type dye, resulting in the coloring of the liquid crystal
display device. On the other hand, in the state shown in FIG. 5B
when a voltage is applied, the N.sub.p liquid crystal having a
positive dielectric anisotropy rises in response to the electric
field and accordingly the p-type dye is perpendicularly aligned.
Therefore, the polarization component Lx is only slightly absorbed,
resulting in the liquid crystal display device being substantially
colorless. The other polarization component Ly included in the
incident light is hardly absorbed by the dichroic dye whether the
state of the voltage is being applied or not being applied.
Accordingly, in the HEILMEIER type guest-host liquid crystal
display device, a polarizer is provided beforehand to remove the
other polarization component Ly for improving the contrast.
Although the guest-host liquid crystal display device shown in FIG.
5 is a transmissive type, a reflective liquid crystal display
device is also known. For example, a reflective guest-host liquid
crystal display device, as shown in FIG. 6, has been proposed, in
which a polarizer is removed on the side of the incident light,
while a quarter-wavelength plate and a reflector are provided on
the emission side. In this device, the polarization directions of
the two polarizing components Lx and Ly which are orthogonal to
each other are rotated by 90 degrees at both incident light and
reflected light paths by the quarter-wavelength plate in order to
exchange the polarizing components with each other. Therefore, in
the off-state (absorption state) shown in FIG. 6A, individual
polarizing components Lx and Ly are absorbed either at the incident
light path or at the reflected light path. In the on-state
(transmission state) shown in FIG. 6B, both polarizing components
Lx and Ly are hardly absorbed. Thus, the utilization efficiency of
the incident light can be improved.
In the transmissive display device shown in FIG. 5, a panel holding
a liquid crystal as an electro-optical material is provided between
a pair of transparent electrodes, and a light source (backlight)
for supplying illumination light is arranged on the rear of the
panel. The image is viewed from the front of the panel. A backlight
is essential to the transmissive type, and, for example, a cold
cathode fluorescent tube or the like is used. Accordingly, from the
viewpoint of the display device as a whole, the backlight consumes
most of the electric power, which is unsuitable for displays of
portable apparatuses. On the other hand, in the reflective type
shown in FIG. 6, a reflector is arranged on the rear of the panel.
External light such as natural light enters from the front and the
image is viewed also from the front of the panel by making use of
the reflected light. Differing from the transmissive type, the
reflective type does not use a light source for supplying
illumination light in the back, resulting in a relatively low rate
of electric power consumption, which is suitable for displays of
portable apparatuses. However, in the reflective display device,
the image cannot be viewed in an environment where external light
is scarce, for example, at night, which remains to be a problem to
be solved.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a
reflective display device provided with an illumination structure
which enables the viewing of an image in a dark environment while
not spoiling the image quality in a bright environment.
A reflective display device, in accordance with the present
invention, includes a panel, a light guide plate and a light source
as fundamental components. The panel includes a transparent first
substrate lying on the side of the external incident light, a
second substrate joined to the first substrate with a predetermined
gap therebetween and lying on the reflection side, an
electro-optical material held in the gap, and an electrode provided
on at least one of the first substrate and the second substrate for
applying a voltage to the electro-optical material. The light guide
plate is composed of a transparent material and arranged on the
outside of the first substrate. The light source is arranged on the
end of the light guide plate and generates illumination light as
required. Notably, the light guide plate normally transmits
external light onto the first substrate and emits the light
reflected from the second substrate, and also as required guides
illumination light onto the first substrate and emits the
illumination light reflected from the second substrate.
Preferably, the light guide plate includes a planar section divided
into bands and an inclined step lying between each band of the
planar section. The thickness of the light guide plate decreases
stepwise from the end where the light source lies toward the front.
The light guide plate reflects the illumination light guided
forward at each step so as to guide it onto the first substrate,
and emits the illumination light reflected from the second
substrate through the planar section. In such a case, the step of
the light guide plate inclines from 40 to 50 degrees toward the
planar section. Further, the panel may use a guest-host liquid
crystal layer, including a liquid crystal as a host to which a
dichroic dye is added as a guest, as the electro-optical material.
In such a case, the panel includes a reflecting layer lying on the
side of the second substrate for scattering and reflecting external
light, and a quarter-wavelength layer provided between the
guest-host liquid crystal layer and the reflecting layer. Or, the
panel may include a polarizing plate provided on the side of the
first substrate and may use a liquid crystal layer, which functions
as a quarter-wavelength plate in response to the state of an
applied voltage, as the electro-optical material. In such a case, a
quarter-wavelength plate is provided between the polarizing plate
and the liquid crystal layer, and the liquid crystal layer includes
a nematic liquid crystal layer having a positive dielectric
anisotropy and a twisted alignment, functions as a
quarter-wavelength plate in the absence of an applied voltage, and
does not function as a quarter-wavelength plate in the presence of
an applied voltage.
Also preferably, the light guide plate and the panel are joined to
each other with a transparent intervening layer therebetween, for
suppressing undesirable reflection of illumination light and
external light at the interface between the light guide plate and
the panel by appropriately setting a refractive index of the
intervening layer. The intervening layer is composed of, for
example, a transparent resin having adhesion. In such a case, the
light guide plate may be provided with a groove on the back surface
thereof for preventing the transparent adhesive resin from leaking
out when the back surface of the light guide plate and the surface
of the panel are joined to each other. Also preferably, the
reflective display device includes a collimating means for
collimating the illumination light radiating from the light source
and leading it perpendicularly onto the end of the light guide
plate. In such a case, the light source is, for example,
semicylindrically formed and arranged facing the end of the light
guide plate, and the collimating means corresponds to a
semicylindrical collimator lens arranged between the light source
and the light guide plate. Also preferably, a polarizing plate is
provided between the light source and the light guide plate for
converting unpolarized light radiating from the light source into
linearly polarized light, leading it onto the light guide plate and
suppressing undesirable scattering of illumination light inside the
light guide plate. In such a case, the polarizing plate converts
illumination light into linearly polarized light which is parallel
to or perpendicular to the light guide plate. On the other hand,
the electro-optical material includes a liquid crystal which can be
controlled in the alignment direction parallel to or orthogonally
to the polarization direction of the illumination light converted
into linearly polarized light. Also preferably, the step of the
light guide plate includes a curved inclined area for reflecting
illumination light diffusively so as to lead it onto the first
substrate. Or, each step of the light guide plate may be formed so
as to have a different angle of inclination for reflecting
illumination light in accordance with the angle of inclination and
leading it onto the first substrate at a different angle. Also
preferably, the light guide plate includes a trapezoidal section
divided into bands and an inclined step lying between each band of
the trapezoidal section, and each band of the trapezoidal section
includes a curved lens area. The light guide plate reflects the
illumination light radiating from the light source at each step so
as to guide it onto the first substrate and emits the illumination
light reflected from the second substrate through the lens area of
each band of the trapezoidal section.
In accordance with the present invention, the light guide plate is
arranged on the surface of the reflective panel, and the light
source is arranged on the end of the light guide plate. In a dark
environment, the light source is turned on and the illumination
light enters into the panel through the light guide plate for
displaying the image. In a bright environment, the light source is
turned off and external light is directly used through the
transparent light guide plate for displaying the image. The light
guide plate is basically transparent and thus it will not prevent
the viewer from seeing the image even in a bright environment. As
described above, in accordance with the present invention, the
light source is turned on only when required, thus the electric
power consumed in the display as a whole can be largely reduced,
which is suitable for displays of portable apparatuses. As an
alternative to the structure in accordance with the present
invention, a flat-type backlight source may be arranged in the rear
of the panel in order to perform an auxiliary illumination in a
dark environment. However, in such a case, it is required either to
provide an opening onto the reflecting layer included in the panel
in order to transmit the illumination light from the back to the
front, or to provide a transflector-type structure to the
reflecting layer. This will lower the reflection efficiency and
sacrifice the brightness of the displayed image in a bright
environment. In the present invention, it is possible to provide an
auxiliary illumination in a dark environment without sacrificing
the display brightness in a bright environment.
Further, in accordance with the present invention, various means
have been tried in order to improve the utilization efficiency of
the illumination light radiating from the light source and to
enhance the display quality. For example, an intervening layer for
matching is provided between the light guide plate and the first
substrate in order to suppress undesirable reflection of
illumination light at the interface between the light guide plate
and the first substrate. Also, a collimating means such as a
collimator is provided between the light source and the end of the
light guide plate in order to lead illumination light efficiently
onto the light guide plate. Also, a polarizing plate is inserted
between the light source and the light guide plate in order to
suppress undesirable scattering of illumination light inside the
light guide plate. Further, since the step of the light guide plate
is formed as a curved inclined area, illumination light is
diffusively led onto the first substrate, resulting in an
improvement in the visual characteristics. Or, each step is formed
so as to have a different angle of inclination and leads
illumination light onto the first substrate at a different angle,
and thus the visual characteristics can be improved. Furthermore,
by providing the planar section (trapezoidal section) divided into
bands with a curved lens area on the light guide plate, a microlens
is provided. The microlens can suppress the interference fringes
caused by the periodical prismatic structure of the light guide
plate, enabling a high-quality display.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic partial sectional view which shows a
reflective display device, as a first embodiment of the present
invention, in use in a dark environment;
FIG. 2 is a schematic partial sectional view which shows the
reflective display device as the first embodiment of the present
invention, in use in a bright environment;
FIGS. 3A-3B are schematic representation showing the optical
properties of the light guide plate included in the first
embodiment shown in FIG. 1 and FIG. 2;
FIG. 4 is a schematic partial sectional view which shows a
reflective display device as a second embodiment of the present
invention;
FIGS. 5A-5B are schematic diagrams showing examples of conventional
transmissive display devices in the absense of voltage in the
presence of voltage respectively;
FIGS. 6A-6B are schematic diagrams showing examples of conventional
reflective display devices in off state and on state
respectively;
FIG. 7 is a sectional view which shows a reflective display device
as a third embodiment of the present invention;
FIGS. 8A-8C are schematic diagram showing the light guide plate
used in the third embodiment, wherein FIG. 8A is a plan view, FIG.
8B is a sectional view and FIG. 8C is an enlarged sectional
view;
FIG. 9 is a plan view showing a light guide plate used in a
variation to the third embodiment;
FIG. 10 is a sectional view showing the light guide plate of the
same;
FIG. 11 is a schematic perspective view which shows a reflective
display device provided with the light guide plate shown in FIG. 9
and FIG. 10;
FIG. 12 is a schematic representation which shows a method for
fabricating the reflective display device shown in FIG. 11;
FIG. 13 is a sectional view which shows a reflective display device
as a fourth embodiment of the present invention;
FIG. 14 is an enlarged partial sectional view which shows in detail
the reflective display device as the fourth embodiment of the
present invention;
FIG. 15 is a perspective view which shows the entire structure of
the fourth embodiment;
FIG. 16 is a perspective view which shows the shape of a collimator
lens used in the fourth embodiment;
FIG. 17 is a schematic representation which shows the optical
properties of the light guide plate incorporated in the fourth
embodiment;
FIG. 18 is a partial sectional view which shows a reflective
display device as a fifth embodiment of the present invention;
FIGS. 19A-19B are partial sectional views of the important part of
a reflective display device as a sixth embodiment of the present
invention;
FIG. 20 is a geometric representation for explaining the sixth
embodiment;
FIG. 21 is a partial sectional view showing a variation to the
sixth embodiment;
FIG. 22 is a schematic diagram showing a typical structure of the
light guide plate used in the reflective display device in
accordance with the present invention;
FIG. 23 is a schematic diagram showing the usage and the properties
of the light guide plate shown in FIG. 22;
FIG. 24 is a sectional view showing a light guide plate as the
important part of a reflective display device as a seventh
embodiment of the present invention;
FIG. 25 is a schematic diagram showing the usage and the properties
of the light guide plate shown in FIG. 24;
FIG. 26 is a schematic partial sectional view which shows a
reflective display device as an eighth embodiment of the present
invention;
FIG. 27 is a schematic diagram which explains the function of the
reflective display device as the eighth embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred embodiments of the present invention will be
explained with reference to the attached drawings.
FIG. 1 is a schematic partial section view which shows a reflective
display device as a first embodiment of the present invention. As
shown in the drawing, the reflective display device includes a
panel 0, a light guide plate 20 and a light source 30 as
fundamental components. The panel 0 comprises a transparent first
substrate 1 lying on the side of the external incident light, a
second substrate 2 joined to the first substrate 1 with a
predetermined gap therebetween and lying on the reflection side, an
electro-optical material held between both substrates 1 and 2, and
electrodes 10 and 11 provided on the first substrate 1 and the
second substrate 2 respectively for applying a voltage to the
electro-optical material. The light guide plate 20 is composed of
the injection-molded piece of a transparent material, for example,
an acrylic resin, and arranged on the outside of the first
substrate 1. Further, in accordance with the embodiment of the
present invention, although the light guide plate 20 and the first
substrate 1 are separately formed, they may be integrally molded.
The light source 30 is arranged on the end of the light guide plate
20 and generates illumination light as required. The light source
30 is composed of, for example, a cold cathode fluorescent tube,
and is a so-called edge light. In order to improve the illumination
efficiency of the edge light, a reflecting mirror 31 is provided
behind the cylindrical light source 30. In such a structure, the
light guide plate 20 normally transmits external light onto the
first substrate 1 and emits the external light reflected from the
second substrate 2, and also as required guides illumination light
onto the first substrate 1 and emits the illumination light
reflected from the second substrate 2.
In accordance with the embodiment of the present invention, the
light guide plate 20 includes a planar section 22 divided into
bands, and an inclined step 21 lying between each band of the
planar section 22. The thickness of the light guide plate decreases
stepwise from the end where the light source 30 lies toward the
front. The light guide plate 20 totally reflects the illumination
light directed forward at each step 21 so as to guide it onto the
first substrate 1, and emits the illumination light reflected from
the second substrate 2 through each band of the planar section 22.
The step 21 of the light guide plate 20 inclines from 40 to 50
degrees toward the planar section 22. In the drawing, the angle of
inclination is shown as .theta.. FIG. 1 shows the reflective
display device in use in a dark environment, where the light source
constituting the edge light is turned on. The illumination light
radiating from the light source 30 illuminates the panel 0 through
the light guide plate 20. That is to say, the illumination light
advancing horizontally in the light guide plate is totally
reflected at the step 21 and enters into the panel 0, while the
illumination light reflected from the second substrate 2 is emitted
through the planar section 22 of the light guide plate 20.
The panel 0 includes a guest-host liquid crystal layer 3, as the
electro-optical material, which comprises a liquid crystal 4 as a
host to which a dichroic dye 5 is added as a guest. However, the
present invention is not limited to a guest-host liquid crystal
layer, and other materials can also be used as the electro-optical
material. The panel 0 includes a reflecting layer 8 and a
quarter-wavelength layer 9. The reflecting layer 8 lies on the side
of the second substrate 2 for scattering and reflecting external
light. The quarter-wavelength layer 9 is provided between the
guest-host liquid crystal layer 3 and the reflecting layer 8. The
structure of the panel 0 will be explained in detail, as follows.
The guest-host liquid crystal layer 3 includes a mixture of a
nematic liquid crystal 4 and a dichroic dye 5 and is homogeneously
aligned by upper and lower alignment layers 6 and 7. Also, a
reflecting layer 8 is provided on the side of the second substrate
2 in the gap between the substrates 1 and 2. Further, the
quarter-wavelength layer 9 is provided between the guest-host
liquid crystal layer 3 and the reflecting layer 8. Electrodes 10
and 11 are formed on the sides of the first substrate 1 and the
second substrate 2 respectively for applying a voltage to the
guest-host liquid crystal layer 3. In accordance with the
embodiment of the present invention, the upper electrode 10 is
formed on the inner surface of the first substrate 1 and the lower
electrode 11 is formed on the inner surface of the second substrate
2.
The reflecting layer 8 has a corrugated surface and scatters light.
Accordingly, its paper-white appearance is suitable for the display
background and since it reflects the incident light with a
relatively wide angle range, the viewing angle range is enlarged,
and thus the display is easily viewed as well as the brightness of
the display being increased. In accordance with the embodiment of
the present invention, a transparent flattening layer 12 is
provided between the reflecting layer 8 and the quarter-wavelength
layer 9 for compensating the corrugation. The quarter-wavelength
layer 9 is composed of a polymeric liquid crystal material 13 which
is aligned uniaxially along the surface of the flattening layer 12.
In order to uniaxially align the polymeric liquid crystal material
13, an underlying alignment layer 14 is provided between the
flattening layer 12 and the quarter-wavelength layer 9. The
reflecting layer 8 includes a resin layer 15 having a corrugation
and a metal film 16 formed on the surface thereof, composed of, for
example, aluminum. The resin layer 15 is a photosensitive resin
layer whose corrugation is patterned by means of
photolithography.
The photosensitive resin layer 15 formed on the surface of the
second substrate 2 is composed of, for example, a photo resist,
which is applied to the entire surface of the substrate. It is
exposed to light through a given mask and, for example, is formed
into a cylindrical pattern. Next, by heating to melt, the
corrugation is formed stably. On the surface of the corrugation
formed as described above, a metal film 16, composed of aluminum or
the like having a predetermined thickness and a good reflectance,
is provided. If the depth of the corrugation is set at several
.mu.m, a good light scattering property is obtained and the
reflecting layer 8 will have a white color. On the surface of the
reflecting layer 8, the flattening layer 12 is provided to
compensate the corrugation. It is preferable that the flattening
layer 12 is composed of a transparent organic substance, for
example, an acrylic resin or the like. By providing the flattening
layer 12, the underlying alignment layer 14 can be stably formed
and rubbed. Thereby, the quarter-wavelength layer 9 is precisely
formed. The alignment layer 7 is formed on the quarter-wavelength
layer 9. The alignment layer 7 provided on the side of the second
substrate 2 and the alignment layer 6 provided on the side of the
first substrate 1 align the guest-host liquid crystal layer 3
homogeneously (horizontally). Alternatively, the guest-host liquid
crystal layer 3 may be aligned homeotropically
(perpendicularly).
Next, the operation for performing the black-and-white display by
using the reflective guest-host liquid crystal display device will
be explained briefly. In the absence of an applied voltage, the
nematic liquid crystal 4 is aligned horizontally and the dichroic
dye 5 is similarly aligned. When the illumination light entering
from the upper first substrate 1 advances to the guest-host liquid
crystal layer 3, a component, of the illumination light, having a
plane of vibration which is parallel to the major axes of the
molecules of the dichroic dye 5, is absorbed by the dichroic dye 5.
Another component, having a plane of vibration which is
perpendicular to the major axes of the molecules of the dichroic
dye 5, passes through the guest-host liquid crystal layer 3, and is
circularly polarized by the quarter-wavelength layer 9 provided on
the surface of the lower second substrate 2, and then it is
reflected from the reflecting layer 8. At this stage, the
polarization of the reflected light is reversed, and after passing
through the quarter-wavelength layer 9 again, the component will
have a plane of vibration which is parallel to the major axes of
the molecules of the dichroic dye 5. Since the component is
absorbed by the dichroic dye 5, a substantially black display is
obtained. On the other hand, in the presence of an applied voltage,
the nematic liquid crystal 4 is aligned perpendicularly along the
direction of the electric field, and the dichroic dye 5 is
similarly aligned. The illumination light entering from the upper
first substrate 1 passes through the guest-host liquid crystal
layer 3 without being absorbed by the dichroic dye 5, and is
reflected from the reflecting layer 8 without being substantially
affected by the quarter-wavelength layer 9. The reflected light
passes though the quarter-wavelength layer 9 again and is emitted
without being absorbed by the guest-host liquid crystal layer 3.
Accordingly, a white display is obtained.
FIG. 2 shows the reflective guest-host liquid crystal display
device shown in FIG. 1 in use in a bright environment. In a bright
environment, because of an ample supply of external light such as
natural light, the display is performed by making use of it.
Therefore, the light source 30 is turned off. Thus the electric
power consumed by the display device as a whole can be reduced. The
light guide plate 20 transmits the light entered from the side of
the viewer onto the first substrate 1 and emits the light reflected
from the second substrate 2 through the planar section 22. Since
the light guide plate 20 is basically transparent, it does not
hinder the viewer from seeing the display.
FIG. 3 is a schematic representation showing the optical properties
of the light guide plate 20. As shown in FIG. 3A, the illumination
light horizontally guided from the end of the light guide plate 20
is totally reflected at substantially a right angle from the step
21 and is guided to the side of the panel 0. By setting the angle
of inclination .theta. of the step 21 appropriately, it is possible
to guide the substantially total volume of the illumination light
to the side of the panel 0 without leaking light. The angle of
inclination .theta. depends on the refractive index of the
transparent material constituting the light guide plate 20 and is
generally set in the range from 40 to 50 degrees.
As shown in FIG. 3B, the illumination light guided to the side of
the panel 0 through the step 21 is reflected from the panel 0 and
is emitted through the planar section 22 of the light guide plate
20. In such a case, it is preferable to set the angle of
inclination .theta. of the step 21 so that the angle .PSI. between
the line perpendicular to the planar section 22 of the light guide
plate 20 and the reflected light is smaller than the angle of total
reflection determined by the refractive index of the light guide
plate 20. Thus the illumination efficiency is improved because the
substantially total volume of the illumination light is emitted
through the planar section 22 of the light guide plate 20 without
being totally reflected. Generally, in order to satisfy the above
condition, the angle of inclination .theta. of the step 21 is set
at 40 to 50 degrees.
FIG. 4 is a schematic partial sectional view which shows a
reflective display device as a second embodiment of the present
invention. The embodiment also has a basically flat structure
including a light guide plate deposited on a panel. Notably, the
panel is an active matrix type. An upper substrate 101 lying on the
side of the incident light in contact with the light guide plate is
composed of a transparent material such as glass. On the other
hand, a lower substrate 102 lying on the reflection side is not
necessarily composed of a transparent material. A guest-host liquid
crystal layer 103 is provided between the pair of substrates 101
and 102. The guest-host liquid crystal layer 103 contains mainly a
nematic liquid crystal 104 having a negative dielectric anisotropy
and contains, at a given ratio, a dichroic dye 105. On the inner
surface of the upper substrate 101, a counter electrode 106 and an
alignment layer 107 are provided. A color filter 150 is also
provided on it. The alignment layer 107 is composed of, for
example, a polyimide film and vertically aligns the quest-host
liquid crystal layer 103. On the lower substrate 102, at least, a
switching element composed of a thin film transistor 108, a
reflecting layer 109, a quarter-wavelength layer 110 and a pixel
electrode 111 are provided. The quarter-wavelength layer 110 is
formed on the thin film transistor 108 and the reflecting layer 109
and is also provided with a contact hole 112 which is connected
with the thin film transistor 108. The pixel electrode 111 is
formed on the quarter-wavelength layer by patterning. Therefore, it
is possible to sufficiently impress the electric field to the
guest-host liquid crystal layer 103 between the pixel electrode 111
and the counter electrode 106. The pixel electrode 111 is
electrically connected with the thin film transistor 108 through
the contact hole 112 which passes through the quarter-wavelength
layer 110.
The individual components will be explained in detail, as follows.
In accordance with the embodiment of the present invention, the
quarter-wavelength layer 110 is composed of a polymeric liquid
crystal layer which is aligned uniaxially. In order to uniaxially
align the polymeric liquid crystal layer, an underlying alignment
layer 113 is provided. In order to compensate for the unevenness of
the thin film transistor 108 and the reflecting layer 109, a
flattening layer 114 is provided, and the above-mentioned
underlying alignment layer 113 is formed on the flattening layer
114. The quarter-wavelength layer 110 is also formed on the surface
of the flattening layer 114. In such a case, the pixel electrode
111 is connected with the thin film transistor 108 through the
contact hole 112 passing through the quarter-wavelength layer 110
and the flattening layer 114. The reflecting layer 109 is
fragmented corresponding to the individual pixel electrodes 111.
Each fragmented part is connected with the corresponding pixel
electrode 111 with the same electric potential. Owing to the
structure described above, the quarter-wavelength layer 110 and the
flattening layer 114 provided between the reflecting layer 109 and
the pixel electrode 111 are not impressed with an electric field
unnecessarily. As shown in the drawing, the reflecting layer 109 is
provided with a scattering reflective surface, which prevents the
regular reflection of the incident light, thus improving the image
quality. The alignment layer 115 is formed so as to cover the
surface of the pixel electrode 111 and is in contact with the
guest-host liquid crystal layer 103 for controlling the alignment
thereof. In accordance with the embodiment of the present
invention, the alignment layer 115 together with the facing
alignment layer 107 vertically aligns the guest-host liquid crystal
layer 103. Finally, the thin film transistor 108 has a bottom-gate
structure where a gate electrode 116, a gate insulating film 117,
and a semiconductor thin film 118 are deposited in that order from
the bottom. The semiconductor thin film 118 is composed of, for
example, polycrystalline silicon and the channel area which matches
with the gate electrode 116 is protected with a stopper 119 from
the top. The thin film transistor 108 having the bottom-gate
structure as described above is covered with a layer insulation
film 120. The layer insulation film 120 has a pair of contact
holes, through which a source electrode 121 and a drain electrode
122 are electrically connected with the thin film transistor 108.
The electrodes 121 and 122 are formed by patterning, for example,
aluminum. The drain electrode 122 and the reflecting layer 109 have
the same electric potential. Also, the pixel electrode 111 is
electrically connected with the drain electrode 122 through the
above-mentioned contact hole 112. On the other hand, a signal
voltage is supplied to the source electrode 121.
FIG. 7 is a schematic sectional view which shows a reflective
display device as a third embodiment of the present invention. This
has the same fundamental structure as the first embodiment of the
present invention shown in FIG. 1 and the same reference numerals
are assigned to the corresponding parts. Notably, the light guide
plate 20 and the first substrate 1 of the panel 0 are joined to
each other with a transparent intervening layer 40 therebetween.
The undesirable reflection of illumination light and external light
at the interface between the light guide plate 20 and the first
substrate 1 is suppressed by appropriately setting a refractive
index of the intervening layer 40. The intervening layer 40 may be
composed of, for example, a transparent resin having adhesion. The
transparent resin is applied to the surface of the first substrate
1 of the panel 0 and the light guide plate 20 is bonded thereto.
Since optical matching is required to suppress the undesirable
reflection, the refractive index of the resin constituting the
intervening layer 40 is selected so as to be substantially the same
as the refractive indices of the light guide plate 20 and the first
substrate 1. For example, if the first substrate 1 is composed of
glass, the refractive index of the resin constituting the
intervening layer 40 is set at approximately 1.5. Also, in order
not to trap air bubbles between the light guide plate 20 and the
panel 0 when they are bonded together, the resin preferably has
relatively low viscosity and the viscosity is adjusted, for
example, to approximately 1,000 cp.
The display quality and the processibility were evaluated by
changing the material for the intervening layer 40. First, for
reference, the light guide plate 20 was directly arranged on the
panel 0 with air therebetween. In such a case, when the
illumination light radiating from the light source 30 enters
vertically into the side of the panel 0, it is reflected from the
interface between the lower surface of the light guide plate 20 and
the air layer as well as from the interface between the air layer
and the upper surface of the first substrate 1. The undesirably
reflected light amounts to approximately 10% of the illumination
light. Since the intensity of the undesirably reflected light is
substantially the same as the amount of light reflected from the
second substrate 2 of the panel 0, the display contrast is
extremely lowered. Since the display contrast in this case reached
approximately zero, the image shown on the panel 0 was not clearly
visible. Next, water was introduced as the intervening layer 40
between the light guide plate 20 and the panel 0. That is, water
having a refractive index of 1.33 was used to fill the interface
between the light guide plate 20 and the panel 0 by means of
capillarity so that they were optically joined together. As a
result, the undesirable reflection at the interface between the
light guide plate 20 and the panel 0 decreased extremely and thus a
contrast which was sufficient enough to view the display was
obtained. Further, an ultraviolet curing epoxy resin was used to
fill the interface between the light guide plate 20 and the panel
0. The refractive index of the epoxy resin was 1.56 and most of the
undesirable surface reflection was eliminated. Thereby, a high
level of contrast which was adequate for practical purposes was
obtained. However, the relatively high viscosity of the epoxy
resin, i.e. approximately 5,000 cp, made it difficult to use it to
uniformly fill the interface between the light guide plate 20 and
the panel 0. Further, an ultraviolet curing epoxy resin having a
viscosity of approximately 1,000 cp was used to fill the interface
between the light guide plate 20 and the panel 0. Since the epoxy
resin has a relatively low viscosity and a high refractive index,
it could be used to uniformly fill the interface between the light
guide plate 20 and the panel 0 and it could almost completely
suppress the undesirable reflection.
FIG. 8 shows a specific structure of the light guide plate 20 shown
in FIG. 7, where FIG. 8A is a plan view, FIG. 8B is a sectional
view and FIG. 8C is an enlarged sectional view. The layered light
guide plate 20 is joined to the panel 0 at a lower surface 28 of
the light guide plate 20. At this stage, if a bonding resin adheres
to an end 25, an upper surface 26, or a side 27, the optical
properties will be damaged. Therefore, it is preferable that the
end 25, the upper surface 26 and the side 27 of the light guide
plate 20 are protected with a tape or the like beforehand when the
light guide plate 20 and the panel 0 are bonded together with an
ultraviolet curing resin. By removing the tape after the light
guide plate 20 and the panel 0 have been bonded together through
the radiation of ultraviolet rays, it is possible to prevent the
bonding resin from unnecessarily adhering. After bonding, the light
guide plate 20 and the panel 0 are integrated.
When the light guide plate is provided on the front surface of the
panel, if an air layer intervenes between the light guide plate and
the panel, nearly 10% of the incident light is reflected because a
refractive index between the air and the light guide plate
disagrees with that between the air and the panel. Since the
reflected light does not participate in the electro-optical
switching of the panel, it significantly decreases the contrast of
the reflective display device. Therefore, in the third embodiment
described above, in order to prevent the interfacial reflection,
the light guide plate and the panel are bonded with a transparent
resin which has a refractive index close to those of them. There
is, however, a possibility that an excess adhesive may leak out of
the gap between the light guide plate and the panel when they are
bonded to each other, and if it should stick to other components,
the reflective display device will have a poor appearance. The
structure for preventing the adhesive from leaking out will be
described as follows. FIG. 9 is a plan view of an improved light
guide plate and FIG. 10 is a sectional view of the same. The light
guide plate 20 is fabricated by cutting an acrylic plate which is
90 mm by 120 mm in size and has a thickness of 3.0 mm with a
diamond cutter having an inclination of 135 degrees. Thus, steps 21
having an inclination angle of 45 degrees are formed with a
distance of 200 .mu.m on the surface of the light guide plate 20. A
planar section 22 is formed between the adjacent steps 21. The
planar section 22 inclines slightly and the light guide plate 20 as
a whole has a given thickness. Prior to the cutting fabrication,
two grooves 29 are formed on the back surface of the acrylic plate.
Each groove 29 is parallel to the step 21, and one is arranged on
the side of the light source and another is arranged on the side
opposite the light source. The groove 29 is formed, for example, 1
mm away from the end of the light guide plate 20, and has a width
of, for example, 1 mm, and a depth of, for example, 0.2 mm.
FIG. 11 schematically shows a reflective display device fabricated
by bonding the light guide plate 20 shown in FIG. 9 and FIG. 10 to
a panel 0. An adhesive 40a having a refractive index of 1.50 is
used to bond the back surface of the light guide plate 20 to the
surface of the panel 0. As described above, two grooves 29 are
formed on the back surface of the light guide plate 20, beforehand.
A light source 30 is arranged on the end of the light guide plate
20 and the light source 30 is partially covered with a reflecting
mirror 31. As the light source 30, a semicylindrical cold cathode
fluorescent tube may be used. A step 21 having an inclination of 45
degrees and a planar section 22 are formed on the surface of the
light guide plate 20. A diffusing area 20z is also formed on the
periphery close to the light source 30. The illumination light
radiating from the light source 30 is totally reflected from the
step 21 formed on the surface of the light guide plate 20 and
guided onto the panel 0 lying below the light guide plate 20.
Accordingly, even in a dark environment, the image on the panel 0
can be displayed by the illumination of the light source 30. The
diffusing area 20z is formed for diffusing and absorbing the
oblique components which have a relatively high incident angle in
the illumination light radiating from the light source 30 so that
the intensity of the illumination light to the panel 0 is
uniformed. This embodiment is characterized by forming a groove 29
on the back surface of the light guide plate 20 for preventing the
leakage of the adhesive 40a (a transparent resin having adhesion)
by the groove 29 when bonding the back surface of the light guide
plate 20 and the surface of the panel 0 to each other.
FIG. 12 is a schematic representation which shows a method of the
fabrication in order to bond the light guide plate 20 and the panel
0 to each other. As described above, two grooves 29 are formed on
the back surface of the light guide plate 20, beforehand. Next, an
adhesive 40a is applied by printing or the like to at least one of
the back surface of the light guide plate 20 and the surface of
panel 0. Then, by pressurizing by a pressing roller 90 while the
light guide plate 20 and the panel 0 are superposed, the light
guide plate 20 and the panel 0 are bonded to each other. A heating
treatment is performed to cure the adhesive 40a as required. Thus,
by using the light guide plate 20 provided with the groove 29 on
the back surface, the leakage of the adhesive 40a can be eliminated
when the light guide plate 20 is bonded to the surface of the panel
0, enabling the prevention of the decline of the production yield
in the bonding step.
FIG. 13 is a schematic sectional view which shows a reflective
display device as a fourth embodiment of the present invention.
This has the same fundamental structure as the second embodiment of
the present invention shown in FIG. 4 and the same reference
numerals are assigned to the corresponding parts for facilitating
understanding. Notably, the embodiment is provided with a
collimating means for collimating the illumination light radiating
from the light source 30 and leading it perpendicularly onto the
end of the light guide plate 20. In the present embodiment, the
collimating means corresponds to a collimator lens 50. The light
source 30 is composed of, for example, a semicylindrical
fluorescent tube and is arranged facing the end 25 of the light
guide plate 20. The collimator lens 50 is also semicylindrical and
is arranged between the light source 30 and the light guide plate
20. The collimator lens 50 is stored together with the light source
30 in a cover 30a. As a collimating means, a parabolic reflector
arranged at the rear of the light source 30 may be used, instead of
the collimator lens 50.
FIG. 14 is a partial sectional view which shows an enlarged pixel
of the reflective display device shown in FIG. 13. The light guide
plate 20 is provided on the outer surface of an upper substrate
101. A guest-host liquid crystal layer composed of a nematic liquid
crystal 104 including a dichroic dye 105 is held between the upper
substrate 101 and a lower substrate 102. The guest-host liquid
crystal layer is driven in response to the electrical field
generated between a counter electrode 106 formed on the upper
substrate 101 and a pixel electrode 111 formed on the lower
substrate 102. On the lower substrate 102, a quarter-wavelength
layer 114 for converting polarization and a reflecting layer 109
for scattering are provided. The reflecting layer 109 includes a
metal film 109a composed of, for example, aluminum, which is formed
on a corrugated resin layer 109b. A resin layer 109c is thinly
applied in order to adjust the corrugation of the resin layer 109b.
The pixel electrode 111 is electrically connected to the metal film
109a through an intermediate electrode 112a. The metal film 109a is
fragmented corresponding to the pixel electrode 111 and is
electrically connected to a drain electrode 122 of a thin film
transistor 108. The thin film transistor 108 has a double gate
structure and is provided with a pair of gate electrodes 116. Also,
a supplementary capacitor Cs is connected to the thin film
transistor 108. Gate insulating films 117a and 117b lying between
the gate electrode 116 and the semiconductor thin film 118 have a
two-layer structure. Also, layer insulation films 120a and 120b
have a two-layer structure.
FIG. 15 is a schematic perspective view which shows the entire
structure of the fourth embodiment. The light guide plate 20 is
provided on the panel 100. The cover 30a provided is connected to
the end 25 of the light guide plate 20. A light source such as a
fluorescent tube and a collimator lens are stored in the cover 30a.
As shown in FIG. 16, the collimator lens 50 is semicylindrical,
i.e., the shape of a cylinder cut vertically to its ends. The
fluorescent tube is arranged facing and parallel to the curved
surface of the collimator lens. The flat surface of the collimator
lens 50 comes into contact with the end 25 of the light guide plate
20.
FIG. 17 is a schematic representation of the light guide plate 20,
showing the optical properties. As shown in the drawing, the light
guide plate 20 has a larger thickness at the side which comes into
contact with the collimator lens 50 and the thickness decreases
toward the front, being a so-called wedge-shape. A minute striped
groove is formed on the inclined upper surface of the light guide
plate 20 and corresponds to a step 21. The illumination light
radiating from the light source 30 is collimated with the
collimator lens 50 and is totally reflected from each step 21 so as
to efficiently enter into a panel (not shown in the drawing). The
light reflected from the panel is emitted toward the side of the
viewer mainly through the planar section 22. The angle of
inclination of the step 21 is set at 42 degrees. When the light
guide plate 20 is composed of glass, the refractive index is 1.5.
The collimated illumination light is totally reflected from the
step 21 and enters into the panel. When air is intervened at the
interface between the light guide plate 20 and the panel, the
incident angle of illumination light toward the panel is 4.5
degrees. Or, when the light guide plate 20 is composed of a
transparent resin material having a refractive index of 1.4, the
angle of total reflection is 45 degrees. If the angle of
inclination of the step 21 is equally set at 45 degrees, the
illumination light collimated with the collimator lens 50 is
totally reflected from the step 21 and enters into the panel
substantially perpendicularly. Thus, the dichroic ratio of the
guest-host liquid crystal layer can be effectively reflected in the
display contrast. The light guide plate having the structure
described above can be fabricated inexpensively if it is processed
with a resin material by using, for example, a stamper. Also, if
the alignment pitch of the step 21 is designed in accordance with
the pixel alignment pitch in the side of the panel, the moire
appearing between them can be minimized.
FIG. 18 is a schematic partial sectional view which shows a
reflective display device as a fifth embodiment of the present
invention. Basically, this embodiment is the same as the first
embodiment shown in FIG. 1, and the same reference numerals are
assigned to the corresponding parts for facilitating understanding.
Notably, the embodiment includes a polarizing plate 60 provided
between the light source 30 and the light guide plate 20 for
converting the unpolarized illumination light which is radiating
from the light source 30 into linearly polarized light and leading
it onto the end 25 of the light guide plate 20. The structure
described above enables suppression of the undesirable scattering
of illumination light inside the light guide plate 20, resulting in
the improvement of the display contrast. Preferably, the polarizing
plate 60 converts illumination light into linearly polarized light
which is parallel to or perpendicular to the light guide plate. On
the other hand, the electro-optical material held in a panel 0
comprises a guest-host liquid crystal layer 3 including a liquid
crystal. The liquid crystal is controlled in the alignment
direction parallel to or orthogonally to the polarization direction
of the illumination light converted into linearly polarized light,
which can enhance the dichroic ratio of the guest-host liquid
crystal layer 3, resulting in the improvement of the display
contrast. When the step 21 is formed on the light guide plate 20 by
cutting operations or the like, a residual distortion generally
occurs inside the light guide plate 20. Thereby, uniaxial
anisotropy occurs inside the light guide plate 20, creating an area
which causes double refraction. If illumination light enters into
the double-refraction area, it is scattered. Since the scattered
light is not related with the optical switching of the panel 0, it
lowers the display contrast of the panel 0. Therefore, the
polarizing plate 60 is provided between the light source 30 and the
light guide plate 20 in order to improve the contrast. In view of
the residual distortion of the light guide plate 20, the scattered
light reaches minimum when the linear polarization direction of
illumination light is parallel to or perpendicular to the light
guide plate 20. Also, by controlling the alignment direction of the
liquid crystal parallel to or orthogonally to the linearly
polarized light, the switching effect of the liquid crystal can be
maximized, and thus the display contrast can be improved.
FIG. 19 shows a reflective display device as a sixth embodiment of
the present invention. FIG. 19A shows the shape of the light guide
plate which is the most important part of the embodiment, and FIG.
19B shows the shape of another light guide plate in comparison with
the embodiment. As shown in FIG. 19A, the step 21 in accordance
with the embodiment includes a curved inclined area and reflects
illumination light diffusively so as to lead it onto the first
substrate of the panel (not shown in the drawing). In the example
shown in the drawing, each step 21 has a convex face and the angle
of inclination ranges from 40 to 50 degrees. The height of the step
is 6 .mu.m and the alignment pitch between the steps 21 is 118
.mu.m. However, these dimensions are just one example. In the
present embodiment, the step 21 has a curvature and the angle of
incidence of illumination light toward the panel is changed
continuously in the range of .+-.5 degrees. That is, the angle
between the tangent line of the convex face and the surface of the
planar section 22 ranges from 40 to 50 degrees. Thus, the scattered
illumination light enters into the panel and the frontal reflection
is suppressed. Therefore, the glare observed when the image is
viewed from the front can be controlled.
In the example to be compared, shown in FIG. 19B, the step has a
flat surface having an inclination of 45 degrees. The illumination
light radiating from the light source advances inside the light
guide plate 20 parallel to the bottom face and is totally reflected
at the step 21 having an inclination of 45 degrees so as to
perpendicularly enter into the panel which is arranged directly
below. In such a case, if the image is viewed from the front of the
panel, the parallel illumination light perpendicularly entering and
the light reflected from the panel interfere with each other,
resulting in glare in the display.
FIG. 20 is a schematic view of a cutout of the curved step 21. The
curved surface of the step 21 lies on a circle with radius R=48.6
.mu.m. By cutting out a circular arc at 31.25 .mu.m from the origin
both in the X and Y axes, a curved surface having an angle of
inclination varying in the range from 40 to 50 degrees is obtained.
In order to form the step having a circular arc like this, for
example, a wedged surface of the light guide plate may be cut into
a stripe with a diamond cutter. Or, after cutting a master disc
with a diamond cutter, it may be used as a stamper to mass-produce
the light guide plate.
FIG. 21 shows a variation to the sixth embodiment. In this example,
steps 21a, 21b, 21c and 21d are formed with different angles of
inclination and reflect illumination light according to the angles
of inclination and lead it onto the side of the panel with
different angles. In such a structure, the same effect as the sixth
embodiment can be obtained.
As described above, the reflective display device does not consume
much electric power and is expected to be used as a display for
information terminals. However, differing from a transmissive
display device which is provided with a backlight, a reflective
display device does not allow an image to be viewed in a dark
environment. In order to solve this problem, in the present
invention, a light guide plate is used. FIG. 22 shows a typical
structure of the light guide plate in accordance with the present
invention. As has been frequently explained, the light guide plate
20 is arranged on the surface of the glass substrate in the front
side of the reflective panel. The light guide plate 20 includes,
for example, the step 21 having an area with an inclination angle
of 45 degrees and the planar section 22 which is parallel to the
glass substrate of the panel, and corresponds to a prism sheet
having a periodical structure.
FIG. 23 is a schematic diagram showing the usage of the prism sheet
shown in FIG. 22. A light source composed of, for example, a cold
cathode fluorescent tube is arranged near the end 25 of the light
guide plate 20. The illumination light radiating from the cold
cathode fluorescent tube horizontally enters through the end 25 and
is almost totally reflected perpendicularly downward at the step 21
having an inclination of 45 degrees. The reflected light can
illuminate the reflective panel 0 from the front side. In a bright
environment, the screen is viewed by using external light, while in
a dark environment, the screen can be viewed by illuminating the
panel 0 with the cold cathode fluorescent tube turned on. By using
the cold cathode fluorescent tube and the light guide plate in this
way, the reflective display device which is usable in any
environment while maintaining a low consumption of electric power
can be obtained. However, when the light guide plate 20 shown in
FIG. 18 is provided on the front surface of the reflective panel 0,
interference fringes may occur owing to the periodical structure of
the light guide plate (prism sheet) depending on the circumstance,
making it difficult to view the screen. When the illumination light
radiating from the light source is reflected perpendicularly
downward, it is diffracted owing to the periodical structure of the
light guide plate 20, generating first-order light, second-order
light, and so on, in addition to zero-order light. The illumination
light reflected from the second substrate 2 passes through the
light guide plate 20 again, however, it is diffracted again and the
zero-order light and the first-order light, etc. are generated. The
zero-order light and the first-order light caused by diffraction
which has occurred twice interfere with each other and bright and
dark fringes overlapping on the screen may be viewed. This may
slightly spoil the good view of the display. In particular, when
the screen is viewed from the front side of the panel 0 obliquely
opposite to the light source, the interference fringes become
distinct. That is, if the eyes of the viewer slant toward the
opposite side to the inclined area of the step 21, the interference
fringes become distinct. As the slant of the eyes of the viewer
becomes larger, the interference fringes become more distinct.
FIG. 24 shows a seventh embodiment of the present invention,
presenting the structure of the light guide plate for improving the
defect described above. As shown in the drawing, the light guide
plate 20 includes a trapezoidal section 22m divided into bands and
an inclined step 21 lying between each band of the trapezoidal
section. The trapezoidal section 22m shown in FIG. 24 corresponds
to the planar section 22 shown in FIG. 22. In the present
embodiment, each trapezoidal section includes a curved lens area
instead of a flat area. The light guide plate 20 reflects the
illumination light radiating from the light source at each step 21
so as to guide it onto the first substrate and also emits the
illumination light reflected from the second substrate through the
lens area of each trapezoidal section 22m. The surface of the light
guide plate 20 is, for example, 90 mm.times.120 mm in size. The end
25 of the light guide plate 20 facing the light source has a
thickness of, for example, H=3.2 mm. The thickness decreases
stepwise moving away from the light source and the tip has a
thickness of, for example, 0.2 mm. The light guide plate 20 is
obtained by processing, for example, a transparent acrylic plate.
The step 21 can be formed by mechanically processing the acrylic
plate with a diamond cutter having an inclination of 135 degrees.
The light guide plate processed like this includes a base of the
acrylic plate, the step 21 having an inclination of 45 degrees, and
the trapezoidal section 22m lying between each step. The
arrangement distance L between the adjacent steps 21 is, for
example, 120 .mu.m. Notably, when the trapezoidal section 22m is
processed, the acrylic plate is cut with a diamond cutter being
concaved to a radius R=1 mm. By using such a cutter, the surface of
the trapezoidal section 22m is processed into a curved lens area.
In the end, the surface structure of the light guide plate 20
includes a microprism corresponding to each step 21 and a microlens
corresponding to each trapezoidal section 22m. By using a light
guide plate having such a structure, it is possible to perform the
front illumination without causing interference fringes to the
reflective panel.
FIG. 25 is a schematic diagram showing the usage and the properties
of the light guide plate shown in FIG. 24. The light guide plate 20
is fixed on the front surface of the reflective panel 0 through the
intervening layer 40 composed of a transparent adhesive or the
like. As described above, in the light guide plate 20, the step 21
having an inclination of 45 degrees is arranged with a distance of,
for example, 120 .mu.m. The trapezoidal section 22m is provided
between the adjacent steps 21. In the drawing, for facilitating
understanding, the properties of the trapezoidal section 22m are
schematically shown using a microlens ML. A light source (not shown
in the drawing) composed of a cold cathode fluorescent tube or the
like is provided adjacently to the end 25 of the light guide plate
20. The illumination light radiating from the cold cathode
fluorescent tube is reflected perpendicularly downward at each step
21 and illuminates the panel 0. The illumination light is reflected
from the second substrate 2 and passes through the light guide
plate 20 to reach the viewer. As described above, in a dark
environment, the panel 0 is viewed by making use of the
illumination light of the cold cathode fluorescent tube. Further,
the adhesive used for the intervening layer 40 may be selected
from, for example, a transparent resin material having a refractive
index of 1.50 in order to improve the optical coupling of the light
guide plate 20 and the panel 0. Also, a deflector may be used in
order to improve the illumination efficiency of the cold cathode
fluorescent tube.
Notably, the microlens ML condenses the parallel diffracted light,
such as the zero-order light and the first-order light emitting
from the trapezoidal section 22m periodically arranged into the
focus of the microlens ML. Accordingly, the viewer who may be
present at substantially an infinite position is not directly
affected by the diffracted light. By using the light guide plate
provided with the microlens, the influence of the interference
fringes can be eliminated. In the structure of the light guide
plate shown in FIG. 18, the planar section 22 does not include a
collimating means, therefore, the parallel diffracted light
emitting from the panel interferes at the infinite point, resulting
in bright and dark fringes. By contrast, in the light guide plate
shown in FIG. 20 and FIG. 21, because of the properties of the
microlens ML, bright and dark fringes are formed at a short
distance right above the panel 0 by several millimeters.
Accordingly, the bright and dark fringes are not at all observed by
the viewer who is present at an appropriate distance and a good
quality display is obtained. As described above, in accordance with
the present embodiment, the light guide plate 20 provided with the
microlens in addition to the microprism can cancel the interference
fringes caused by the periodical prism structure, and a good
quality display is obtained even in the case of the front
illumination.
FIG. 26 is a schematic partial sectional view which shows a
reflective display device as an eighth embodiment of the present
invention. The same reference numerals are assigned to the parts
corresponding to those of the first embodiment shown in FIG. 1 for
facilitating understanding. In this embodiment a Twist
Nematic-Electrically Controlled Birefringence (TN-ECB) mode liquid
crystal panel is used as the panel 0, while in the first embodiment
a guest-host mode liquid crystal panel is used. As shown in the
drawing, the reflective display device includes a light guide plate
20 and a panel 0. A step 21 and a planar section 22 are formed on
the surface of the light guide plate 20. The back surface of the
light guide plate 20 is deposited on the surface of the panel 0. A
polarizing plate 70 and a quarter-wavelength plate 80 are provided
on the surface of the panel 0. The panel 0 includes a first
substrate 1 composed of transparent glass or the like lying on the
side of external incident light joined to a second substrate 2
lying on the reflection side with a predetermined gap therebetween.
A nematic liquid crystal layer 3a is held in the gap between both
the substrates 1 and 2. The liquid crystal molecules 4 are
twistedly aligned by the upper and lower alignment layers 6 and 7.
Electrodes 10 and 11 are provided on the inner surfaces of the
substrates 1 and 2, respectively, for applying a voltage to the
nematic liquid crystal layer 3a of each pixel. In this embodiment,
the electrode 10 provided on the side of the first substrate 1 is
patterned into a stripe, and the electrode 11 provided on the side
of the second substrate 2 is also formed into a stripe. Both
electrodes 10 and 11 are arranged orthogonally to each other and
pixels are delimited by the crossing parts, which is a so-called
"passive matrix" type. The polarizing plate 70 and the
quarter-wavelength plate 80 are arranged on the side of the first
substrate 1 of the panel 0. The reflective liquid crystal display
device having such a structure is a TN-ECB type and has a so-called
"normally white" mode. That is, in the absence of an applied
voltage, the nematic liquid crystal layer 3a functions as a
quarter-wavelength plate by maintaining a twisted alignment and
performs a white display by transmitting external light in
cooperation with the polarizing plate 70 and the quarter-wavelength
plate 80. In the presence of an applied voltage, the nematic liquid
crystal layer 3a does not function as a quarter-wavelength plate by
shifting to a vertical alignment and performs a black display by
intercepting external light in cooperation with the polarizing
plate 70 and the quarter-wavelength plate 80.
In reference to FIG. 26, each component will be described in
detail. As mentioned above, the polarizing plate 70 is provided on
the surface of the first substrate 1 of the panel 0. The
quarter-wavelength plate 80 is provided between the polarizing
plate 70 and the first substrate 1. The quarter-wavelength plate 80
is composed of, for example, a uniaxially stretched polymeric film,
and a phase difference by a quarter-wavelength arises between an
ordinary ray and an abnormal ray. The optical axis (uniaxial
anisotropic axis) of the quarter-wavelength plate 80 is arranged at
an angle of 45 degrees toward the polarization axis (transmission
axis) of the polarizing plate 70. The external light is converted
into a linearly polarized light by passing through the polarizing
plate 70. The linearly polarized light is converted into a
circularly polarized light by passing through the
quarter-wavelength plate 80. It is again converted into a linearly
polarized light by passing through the quarter-wavelength plate. In
such a case, the polarization direction rotates by 90 degrees from
the original polarization direction. As described above, the
quarter-wavelength plate in combination with the polarizing plate
can rotate the polarization direction, which is used for
displaying.
The panel 0 uses the nematic liquid crystal layer 3a, as the
electro-optical material, which is basically horizontally aligned
and has a positive dielectric anisotropy. The nematic liquid
crystal layer 3a functions as a quarter-wavelength plate by
appropriately setting its thickness. In this embodiment, the
refractive index anisotropy .DELTA.n of the nematic liquid crystal
layer 3a is approximately 0.7 and the thickness of the nematic
liquid crystal layer 3a is approximately 3 .mu.m. Therefore, the
retardation .DELTA.n.multidot.d of the nematic liquid crystal layer
3a is from 0.2 to 0.25 .mu.m. By twistedly aligning the nematic
liquid crystal molecules 4 as shown in the drawing, the
above-mentioned retardation value will reach approximately 0.15
.mu.m (150 nm). The value is approximately one fourth of the
central wavelength of the external light (approximately 600 nm),
thus enabling the nematic liquid crystal layer 3a to optically
function as a quarter-wavelength plate. A predetermined twisted
alignment can be obtained by sandwiching the nematic liquid crystal
layer 3a between the upper and lower alignment layers 6 and 7. The
liquid crystal molecules 4 align along the rubbing direction of the
alignment layer 6 on the side of the first substrate 1 and the
liquid crystal molecules 4 align along the rubbing direction of the
alignment layer 7 on the side of the second substrate 2. By
shifting the rubbing direction of the alignment layer 6 from that
of the alignment layer 7 by 60 to 70 degrees, a predetermined
twisted alignment can be obtained.
A reflecting layer 8 is formed below the electrode 11 on the side
of the second substrate 2. The reflecting layer 8 has a corrugated
surface and scatters light. Accordingly, its paper-white appearance
is suitable for the display background and since it reflects the
incident light with a relatively wide angle range, the viewing
angle range is enlarged, and thus the display is easily viewed as
well as the brightness of the display being increased. A
transparent flattening layer 12 is provided between the reflecting
layer 8 and the electrode 11 for compensating the corrugation. The
reflecting layer 8 includes a resin layer 15 having a corrugation
and a metal film 16 formed on the surface thereof, composed of, for
example, aluminum. The resin layer 15 is a photosensitive resin
layer whose corrugation is patterned by means of photolithography.
The photosensitive resin layer 15 is composed of, for example, a
photo resist and applied to the entire surface of the substrate. It
is exposed to light through a given mask and, for example, is
formed into a cylindrical pattern. Next, by heating to melt, the
corrugation is formed stably. On the surface of the corrugation
formed as described above, a metal film 16, composed of aluminum or
the like having a predetermined thickness and a good reflectance,
is provided. If the depth of the corrugation is set at several
.mu.m, a good light scattering property is obtained and the
reflecting layer 8 will have a white color. On the surface of the
reflecting layer 8, the flattening layer 12 is provided to
compensate the corrugation. It is preferable that the flattening
layer 12 is composed of a transparent organic substance, for
example, an acrylic resin or the like. By intervening the
flattening layer 12, the electrode 11 and the alignment layer 7 are
formed stably.
With reference to FIG. 27, the performance of the reflective
display device shown in FIG. 26 will be described in detail. In the
drawing, (OFF) shows the state in the absence of an applied voltage
and (ON) shows the state in the presence of an applied voltage. As
shown in (OFF), the reflective display device includes the
polarizing plate 70, the quarter-wavelength plate 80, the nematic
liquid crystal layer 3a and the reflecting layer 8 deposited in
that order from the side of the viewer. The polarization axis
(transmission axis) of the polarizing plate 70 is shown by 70P. The
optical axis 80S of the quarter-wavelength plate 80 and the
transmission axis 70P form an angle of 45 degrees. Also, the
alignment direction 3R of the liquid crystal molecules 4 on the
side of the first substrate is parallel to the polarization axis
(transmission axis) 70P of the polarizing plate 70.
The incident light 201 is converted into a linearly polarized light
202 by passing through the polarizing plate 70. Its polarization
direction is parallel to the transmission axis 70P, and hereinafter
it is referred to as a "parallel linearly polarized light". The
parallel linearly polarized light 202 is converted into a
circularly polarized light 203 by passing through the
quarter-wavelength plate 80. The circularly polarized light 203 is
converted into a linearly polarized light by passing through the
nematic liquid crystal layer 3a which functions as a
quarter-wavelength plate. However, the polarization direction of
the linearly polarized light is rotated by 90 degrees and is
orthogonal to the parallel linearly polarized light 202.
Hereinafter, it is referred to as an "orthogonal linearly polarized
light". The orthogonal linearly polarized light 203 is converted
into a circularly polarized light 204 because it again passes
through the nematic liquid crystal layer 3a which functions as a
quarter-wavelength plate after being reflected from the reflecting
layer 8. The circularly polarized light 204 is converted into a
parallel linearly polarized light 205 the same as before because it
yet again passes through the quarter-wavelength plate 80. The
parallel linearly polarized light 205 eventually becomes an
emitting light 206 after passing through the polarizing plate 70 to
reach the viewer, and thus a white display is obtained.
In the state shown in (ON), in the presence of an applied voltage,
the liquid crystal molecules 4 have a vertical alignment instead of
a twisted alignment and do not function as a quarter-wavelength
plate. The external light 201 passing through the polarizing plate
70 is converted into a parallel linearly polarized light 202. The
parallel linearly polarized light 202 is converted into a
circularly polarized light 203 by passing through the
quarter-wavelength layer 80. The circularly polarized light 203
passes through the nematic liquid crystal layer 3a as a circularly
polarized light, is reflected by the reflecting layer 8, and
reaches the quarter-wavelength layer 80 as a circularly polarized
light 204a. The quarter-wavelength layer 80 converts the circularly
polarized light 204a into an orthogonal linearly polarized light
205a. Since the orthogonal linearly polarized light 205a cannot
pass through the polarizing plate 70, a black display is
obtained.
As described above, in accordance with the present invention, a
light guide plate is arranged on a reflective panel, and a light
source for performing an auxiliary illumination is arranged on the
end of the light guide plate. The light guide plate normally
transmits external light onto the panel and emits the external
light reflected from the panel, and also, as required, guides
illumination light onto the panel and emits the illumination light
reflected from the panel. In a dark environment, although the panel
is a reflective type, the image can be viewed by turning the light
source on. On the other hand, in a bright environment which has an
abundant amount of external light, electric power can be saved by
turning the light source off. Also, in accordance with the present
invention, the light guide plate and the panel are joined to each
other with a transparent intervening layer therebetween in order to
suppress undesirable reflection of illumination light and external
light at the interface between the light guide plate and the panel
by appropriately setting a refractive index of the intervening
layer. Since matching of the refractive index between the light
guide plate and the panel is obtained, in an environment which has
an abundant amount of external light, for example, in the daytime,
external light is efficiently led into the panel, and also when
illumination light is required, for example, at night, the
undesirable reflection can be almost completely suppressed. By
joining the light guide plate and the panel with an adhesive, they
can be integrated. In particular, by providing a groove on the back
surface of the light guide plate, the adhesive resin can be
prevented from leaking out when the back surface of the light guide
plate and the surface of the panel are joined to each other. Also,
in accordance with the present invention, a collimating means like
a collimator lens is employed in order to collimate the
illumination light radiating from the light source and lead it
perpendicularly onto the end of the light guide plate, and thus the
utilization efficiency of the illumination light can be greatly
improved. Also, in accordance with the present invention, by
providing a polarizing plate between the light source and the light
guide plate, the undesirable scattering of the illumination light
inside the light guide plate can be suppressed, and thus the
display contrast can be improved. In addition, by leading linearly
polarized light into the panel, the optical switching properties of
the panel become more efficient, resulting in the improvement of
the display contrast. Also, in accordance with the present
invention, the step of the light guide plate includes a curved
inclined area for reflecting illumination light diffusively and
leading it into the side of the panel. Thus, the intensity of the
frontal reflected light can be lowered and the glare of the display
screen can be controlled. Also, in accordance with the present
invention, the light guide plate includes a trapezoidal section
divided into bands and an inclined step lying between each
trapezoidal section, and also each trapezoidal section includes a
curved lens area. Thus, the interference fringes caused by the
periodical structure of the light guide plate can be controlled,
and the quality of the display screen can be improved.
* * * * *